(1) Field of the Invention
The present invention relates to a non-sintered nickel electrode and a manufacturing method thereof, and further to an alkaline storage cell which includes the non-sintered nickel electrode.
(2) Related Art
Nickel electrodes for use in alkaline storage cells are classified into sintered type and non-sintered type.
A sintered type nickel electrode is manufactured by repeating the following two soaking operations: firstly a porous sintered nickel substrate used as an active material holder is soaked in a solution of an acid nickel salt, such as nickel nitrate, so as to fill the pores with nickel salt, and secondly the substrate is soaked in an alkali solution so as to convert the nickel salt into nickel hydroxide.
In contrast, a non-sintered type nickel electrode is manufactured by applying a nickel active material which is manufactured separately onto an electrode substrate. One well-known method of manufacturing such an electrode is to mix a nickel active material which is mainly composed of nickel hydroxide with a conductive agent, a binder, water, and the like into a paste, to apply this paste onto an electrode substrate such as a punching metal, and to dry the coated substrate. Another well-known method is to fill an electrode substrate which is made from an open pore metal or a sintered metallic fiber with a nickel active material slurry.
Of these two types, sintered type nickel electrodes have a disadvantage that the manufacturing process is complex since the soaking operations must be repeated several times to obtain a sufficient amount of active material. Another disadvantage is that the substrate has low porosity, which sets limits on the capacity of the electrode.
Non-sintered type nickel electrodes, on the other hand, are free from these drawbacks, while they are inferior to the sintered type nickel electrodes in the utilization factor of nickel hydroxide as an active material.
Through various studies on the utilization factor of active materials, it is now known that the addition of a highly conductive high-order cobalt compound to the active material used in a non-sintered nickel electrode leads to an improvement in efficiency.
For example, Japanese Laid-open Patent Application No. 1-200555 describes a technique of manufacturing highly-conductive high-order cobalt compound layers such as CoOOH and Co2O3, by forming cobalt hydroxide layers over the surfaces of nickel hydroxide active material particles and then subjecting the cobalt hydroxide layers to heat treatment in the presence of alkali.
The utilization factor of active materials can be improved by adding an active material containing a highly-conductive high-order cobalt compound to a nickel electrode because the high-order cobalt compound layers form a conductive network within the electrode.
However, such non-sintered nickel electrodes still have a drawback that their capacity is greatly decreased during an over-discharge operation.
A first object of the present invention is to provide a non-sintered type nickel electrode which has a high utilization factor of a nickel hydroxide active material and effectively restricts a capacity decrease during an over-discharge operation.
A second object of the present invention is to provide a manufacturing method of such a non-sintered type nickel electrode.
A third object of the present invention is to provide an alkali storage cell which includes such a non-sintered type nickel electrode.
The first object can be achieved by a non-sintered nickel electrode supplied with an active material containing nickel hydroxide, a cobalt compound, and at least one of zinc, cadmium, magnesium, and calcium. At least one of zinc, cadmium, magnesium, and calcium is added in a form of a solid solution to the nickel hydroxide, and the cobalt compound is formed into layers over surfaces of particles of the nickel hydroxide. The cobalt compound has an oxidation number of larger than 2 and a disordered crystal structure.
The third object can be achieved by using the above-constructed nickel electrode, an alkali electrolyte, a separator which is mainly composed of unwoven polyolefin resin fiber, and a negative electrode which is composed of a MmNi5 system hydrogen-absorbing alloy as components of a cell.
In the nickel electrode which is constructed as explained above, the utilization factor of active materials is remarkably improved. This improvement results from the fact that the high-order cobalt compound which has an oxidation number of larger than 2 and a disordered crystal structure has an extremely high conductivity, so that a conductive network is formed in the electrode.
In the nickel electrode which is constructed as explained above, a capacity decrease to be caused by an over-discharging operation is also restrained. This restraint seems to result from the following:
In a conventional nickel active material having high-order cobalt compound layers on the surfaces, the cobalt compound penetrates into the particles of the nickel active material when the cell is being over-discharged. As a result, the amount of cobalt on the surfaces lessens, decreasing the conductive network function within the electrode, thereby decreasing the capacity of the cell.
In contrast, when a solid solution of a metal such as zinc, cadmium, magnesium, and calcium is added to nickel hydroxide powder, the metal works to restrain the penetration of the cobalt compound into the nickel hydroxide power, so that the reduction of the amount of cobalt on the surfaces is restrained when the cell is being over-discharged.
Furthermore, when at least one of these metals is added in the form of being liberated from the nickel hydroxide active material, the chargeability at a high temperature is improved because the oxygen generation potential during a charging operation is shifted to being noble.
There are two methods of manufacturing such an active material.
One method is to mix nickel hydroxide powder containing a solid solution of at least one of zinc, cadmium, magnesium, and calcium with either metallic cobalt or a cobalt compound, and to subject the mixture to heat treatment in the presence of oxygen and alkali. A preferable amount of the metallic cobalt and the cobalt compound is 5 mol% to 14 mol% to the nickel hydroxide. This method is much easier.
The other method is to precipitate a cobalt compound over the surfaces of the nickel hydroxide powder containing a solid solution of at least one of zinc, cadmium, magnesium, and calcium so as to form cobalt compound layers, before subjecting it to the heat treatment in the same conditions. This method allows the cobalt compound layers to be formed more uniformly.
In either method, a preferable concentration of the alkali aqueous solution in the alkali heat treatment would be 15% by weight to 40% by weight, and a preferable temperature of the alkali heat treatment would be 50xc2x0 C. to 150xc2x0 C.
In addition, the use of an alkali solution including lithium ions for the alkali heat treatment contributes to the restriction of the capacity decrease to be caused by an over-discharging operation.
Therefore, an alkali storage cell including the nickel electrode of the present invention has a high utilization factor of active materials and restricts a capacity decrease during an over-discharge operation. In other words, such a cell has a great industrial value because of its large capacity and operational stability.